A direct digital frequency synthesizer (DDFS) is a digitally-controlled signal generator that may vary the output signal frequency over a large range of frequencies, based on a single fixed-frequency precision reference clock. In addition, a DDFS is also phase-tunable. In essence, within the DDFS, discrete amplitude levels are input to a digital-to-analog converter (DAC) at a sampling rate determined by the fixed-frequency reference clock. The output of the DDFS may provide a signal whose shape may depend on the sequence of discrete amplitude levels that are input to the. DAC at the constant sampling rate. The DDFS is particularly well suited as a frequency generator that outputs a sine or other periodic waveforms over a large range of frequencies, from almost DC to approximately half the fixed-frequency reference clock frequency.
A DDFS offers a larger range of operating frequencies and requires no feedback loop, thereby providing near instantaneous phase and frequency changes, avoiding overshooting, undershooting and settling time issues associated with other analog systems. A DDFS may provide precise digitally-controlled frequency and/or phase changes without signal discontinuities.
Polar modulation is related to inphase (I) and quadrature (Q) modulation similar to polar coordinates in the Cartesian coordinate system. For polar modulation, the orthogonal I and Q components of a RF signal may be converted to a phasor representation comprising an amplitude component and a phase component. The combined I and Q signal components may be generated with one phase change and one amplitude change, for example, whereas separate I and Q modulation may require amplitude and phase modulation for each channel, especially for non-constant envelope modulation modes. In addition, the I and Q modulation approach may require good linearity of the power amplifier, often leading to power inefficient designs that suffer from parameter variability due to factors such as temperature. In contrast, polar modulation may allow the use of very efficient and non-linear amplifier designs for non-constant envelope modulation schemes.
Both Bluetooth and WLAN radio devices, such as those used in, for example, handheld wireless terminals, generally operate in the 2.4 GHz (2.4000-2.4835 GHz) Industrial, Scientific, and Medical (ISM) unlicensed band. Other radio devices, such as those used in cordless phones, may also operate in the ISM unlicensed band. While the ISM band provides a suitable low-cost solution for many of short-range wireless applications, it may also have some drawbacks when multiple users operate simultaneously. For example, because of the limited bandwidth, spectrum sharing may be necessary to accommodate multiple users and/or multiple different types of communication protocols. Multiple active users may also result in significant interference between operating devices. Moreover, in some instances, other devices such as microwave ovens may also operate in this frequency spectrum and may produce significant interference or blocking signals that may affect Bluetooth and/or WLAN transmissions.
Oscillators may be utilized in wireless receivers and transmitters to provide frequency conversion, and to provide sinusoidal sources for modulation. The oscillators may operate over frequencies ranging from several kilohertz to many gigahertz, and may be tunable over a set frequency range. A typical oscillator may utilize a transistor with a LC network to control the frequency of oscillation. The frequency of oscillation may be tuned by adjusting the values of the LC resonator. A crystal controlled oscillator (XCO) may be enabled to provide an accurate output frequency, if the crystal is in a temperature controlled environment. A phase locked loop (PLL) may utilize a feedback control circuit and an accurate reference source such as a crystal controlled oscillator to provide an output that may be tunable with a high accuracy. Phase locked loops and other circuits that provide accurate and tunable frequency outputs may be referred to as frequency synthesizers.
Phase noise is a measure of the sharpness of the frequency domain spectrum of an oscillator, and may be critical for many modern wireless systems as it may severely degrade the performance of a wireless system. The phase noise may add to the noise level of the receiver, and a noisy local oscillator may lead to down conversion of undesired nearby signals. This may limit the selectivity of the receiver and the proximity of spacing adjacent channels in a wireless communication system.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.